Mimosa Webworm

Overview

Mimosa webworm (Homadula anisocentra) is a non-native moth that feeds on the leaflets of mimosa trees (Albizia julibrissin), causing them to turn brown and defoliate. However, in the U.S., it primarily affects honey locust (Gleditsia triacanthos), including thornless cultivars. Mimosa webworm was first detected in the U.S. in the 1940’s on mimosa trees planted in Washington, D.C. It is now primarily found throughout the Eastern U.S., Midwest, southeast, and west to Kansas and Oklahoma. It can also be found as far west as California. In the Midwest, there are two generations of webworms that are active from June to September. In some areas in the U.S. there are reportedly three generations and overlap between generations can occur. Note: Mimosa/silk trees are not recommended for planting as they are an invasive species in Missouri and several states in the east and southeast.
 

Life Cycle

The first generation of moths first emerge from overwintered pupal sites approximately early to mid-June. Adults are a steel-gray color, with small black dots along their forewings. Females will lay small, spherical, pearl-colored eggs on leaflets and branches that turn rose-pink just before hatching. Young larvae are yellow green in color and will feed on the foliage in large groups and form a protective webbing around the leaflets. Mature larvae are just over a ½ inch long (15 mm) and are a gray to dark brown color, with five white longitudinal stripes along its body. Larval feeding behavior continues until they pupate inside the webbing in July. The second generation of adult moths appear between late July and mid-August. Females lay eggs within the webbed leaflets, and the larvae produce new webs and feed until September. Just before they pupate, larvae will descend on silk strands to locate overwintering sites, such as bark crevices of host trees, other woody plants, or artificial structures.   
 

Symptoms and Diagnosis

Symptoms of webworm damage begin to appear in June on host species when the first-generation begins feeding, causing severe skeletonization of the leaflets. Larvae feed in large groups and envelop leaflets in thick webbing as a form of protection. As a result, ample amounts of brown frass can be found throughout the webbing. Feeding damage is often first noticed at the ends of branch sections as leaflets begin to turn brown from desiccation. As the second-generation of larvae emerge in July/August, foliar damage becomes more severe as it expands along branches. Entire canopies may appear completely brown or bronzed by August. If disturbed, the larvae may thrash around and drop down on silk strands in defense. Population densities and damage severity can vary from year to year.  

Integrated Pest Management Strategies

1. Live with the problem. Damage can be unsightly, but most healthy trees can withstand repeated years of feeding. Those that have defoliated will leaf out again the following year. 

2. Avoid extensive plantings of honey locust. The thornless cultivar ‘Sunburst’ is reportedly very susceptible to damage by mimosa webworm, while others such as ‘Moraine’, Shademaster’, and ‘Skyline’ and are less susceptible.

3. Apply insecticides. Products containing Bt (Bacillus thuringiensis spp. kurstaki) can be used on trees with a history of repeated, severe infections. Timing is critical, as applications should be made when larvae from the first generation are small, but before webbing becomes too dense. Treatments are not recommended if trees are heavily infested with webbing or damage is severe, as they will be ineffective. Additional broad-spectrum insecticides are available but are not recommended as they may affect non-target insects such as bees and other natural predators. Despite mimosa webworm not being native to the U.S., several parasitoids have been found to target pupae and larvae.

Organic Strategies

Strategies 1 & 2 are strictly organic approaches. For organic approaches to Strategy 3, consult the Organic Materials Review Institute (OMRI™) for appropriate Bacillus thuringiensis (Bt) products. 

Pesticide Disclaimer: 

Always follow the product's label and ensure the product is effective against bagworms. Not following the pesticide label before usage is a violation of federal law.